1. Field of the Invention
The invention relates to impeller shafts and, more particularly, to the construction of such shafts and to a drive system that (a) enables the shafts to be connected or disconnected quickly, (b) reduces the possibility that the shafts will be fractured by excessive torque or vibrational loads, and (c) enables the shafts to be axially adjusted easily.
2. Description of the Prior Art
Impeller shafts are used in a variety of applications such as molten metal pumps, mixers, dispersers, and other equipment. In particular, impeller shafts made of graphite are used in environments that are hostile to shafts made of other materials. An especially desirable use for graphite shafts is that of supporting rotatable impellers in molten metal pumps. Properly treated, graphite is an oxidation-resistant material that enables the shafts to be effective in withstanding attack by corrosive substances such as molten aluminum.
A problem with graphite shafts is that they are not very strong. Consequently, it is difficult to work with them and they must be handled carefully. Another drawback of graphite shafts relates to the technique by which they are connected to drive motors and impellers. It is conventional to connect a graphite shaft to a drive motor by threading the shaft into a coupling connected directly or indirectly to the motor. Unfortunately, a threaded shaft cannot be adjusted axially relative to the coupling because the threaded end of the shaft will be bottomed out upon connection to the coupling. The same problem occurs when an impeller is threaded onto the other end of the shaft. Additionally, after the shaft has been used, torque that has been transmitted to the shaft during use will cause the shaft to be very tightly connected to the coupling and the impeller such that the components cannot be separated easily. In extreme cases, it is necessary to destroy the shaft in order to remove the shaft from the coupling.
An additional problem related to graphite shafts is that they frequently are fractured in use due to excessive torque or vibrational loads that are applied to the shafts. In the particular instance of graphite shafts used in molten metal pumps, it sometimes happens that foreign objects are ingested into the pumps. In this circumstance, an excessive torque load may be applied to the shafts, resulting in catastrophic failure of the shafts, often in the region of the relatively weak threaded connections. Another problem is that threaded shafts can transmit torque only in the direction which tightens the threads. A shoulder must be provided to limit the advancement of the thread into the mating part. This subjects the shaft to a high tensile stress between the thread and the shoulder. Graphite tolerates much lower tensile stress than compressive stress. Failure at the thread-shoulder intersection is, by far, the most common failure location
Desirably, an impeller shaft would be available that would enable the impeller to be easily connected to the shaft with minimal machining of the shaft and the impeller, and which would have superior strength characteristics. Another advantageous feature would be a drive motor-impeller shaft connection that would permit the shaft to connected and disconnected easily from the drive motor without requiring excessive handling o machining of the shaft. Additionally, it would be desirable to have a drive motor-impeller shaft connection that would be stronger than conventional threaded shaft connections, that would resist vibrational loads well, and that would permit the ready replacement of worn or broken parts. Yet additionally, it would be desirable to be able to easily adjust the shaft axially relative to the drive motor.